Ofdma baseband clock synchronization

ABSTRACT

A method for synchronizing baseband clocks in an OFDMA wireless microphone system is disclosed. An example method includes receiving a plurality of pilot subcarriers from an audio transmitter. The method also includes determining a timing offset estimate based on the pilot subcarriers. The method further includes determining a tuning value by passing the timing offset estimate through a proportional-integral controller. The method still further includes determining a modified reference signal by modifying a reference oscillator based on the tuning value. And the method yet further includes controlling (i) an audio sample clock and (ii) an antenna data clock based on the modified reference signal.

TECHNICAL FIELD

This application generally relates to timing and frequencysynchronization for wireless audio systems and devices using orthogonalfrequency division multiple access (OFDMA) for communication,particularly audio devices having an audio sample clock and an antennadata clock.

BACKGROUND

Orthogonal frequency division multiplexing (OFDM) is a method ofencoding digital data on multiple carrier frequencies. Subcarriers aresent together to form a wideband high speed communication link. Thiscommunication link can be used for many purposes, including digitaltelevision and audio broadcasting, DSL internet access, wirelessnetworks, power line networks, and mobile communications.

In an OFDMA audio system, there may be an access point and one or moresubscribers. The subscriber devices must compensate for any frequencyoffset in their respective antenna data clocks relative to the accesspoint, so that frame timing and symbol timing are maintained. Thisallows the subscriber devices to properly receive and transmit data withthe access point.

Existing subscriber devices may include a sample rate conversion blockthat prevents audio distortion introduced by inserting or droppingsamples. The sample rate conversion block, however, requires additionalresources and introduces latency into the audio path.

Accordingly, there is an opportunity for a method and system for OFDMAbaseband clock synchronization that does not require an additionalsample rate conversion block, particularly in the context of highquality audio applications, and thus reduces the resources required andremoves a source of latency in the audio path.

SUMMARY

Embodiments of the present disclosure are intended to alleviate some ofthe above-noted problems by providing methods and systems for lockingthe baseband clocks of the subscribers in a wireless microphone systemto the access point, by generating the respective baseband clocks from acommon reference. The access point (also referred to as an “audiotransmitter”) and each subscriber device (also referred to as “audioreceivers”) all include one or more baseband clocks (i.e., controllingan audio sample clock and/or data sample clock) that are set based onreference oscillators. Embodiments of the present disclosure may includetuning the reference oscillators of each subscriber device based onmeasured phase differences, such that the baseband clocks of all thesubscribers and the access point are in sync.

An example method includes receiving a plurality of pilot subcarriersfrom an audio transmitter. The method also includes determining a timingoffset estimate based on the pilot subcarriers. The method furtherincludes determining a tuning value by passing the timing offsetestimate through a proportional-integral controller. The method stillfurther includes determining a modified reference signal by modifying areference oscillator based on the tuning value. And the method yetfurther includes controlling (i) an audio sample clock and (ii) anantenna data clock based on the modified reference signal.

These and other embodiments, and various permutations and aspects, willbecome apparent and be more fully understood from the following detaileddescription and accompanying drawings, which set forth illustrativeembodiments that are indicative of the various ways in which theprinciples of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a wireless communication system,in accordance with some embodiments of the present disclosure.

FIG. 2 is a simplified signal process flow diagram of an examplesubscriber device of the system of FIG. 1, in accordance with someembodiments of the present disclosure.

FIG. 3 is a simplified signal process flow diagram of the sample timingoffset estimator of FIG. 2, in accordance with some embodiments of thepresent disclosure.

FIG. 4 is a flowchart illustrating an example method, in accordance withsome embodiments of the present disclosure.

DETAILED DESCRIPTION

The description that follows describes, illustrates and exemplifies oneor more particular embodiments of the invention in accordance with itsprinciples. This description is not provided to limit the invention tothe embodiments described herein, but rather to explain and teach theprinciples of the invention in such a way to enable one of ordinaryskill in the art to understand these principles and, with thatunderstanding, be able to apply them to practice not only theembodiments described herein, but also other embodiments that may cometo mind in accordance with these principles. The scope of the inventionis intended to cover all such embodiments that may fall within the scopeof the appended claims, either literally or under the doctrine ofequivalents.

It should be noted that in the description and drawings, like orsubstantially similar elements may be labeled with the same referencenumerals. However, sometimes these elements may be labeled withdiffering numbers, such as, for example, in cases where such labelingfacilitates a more clear description. Additionally, the drawings setforth herein are not necessarily drawn to scale, and in some instancesproportions may have been exaggerated to more clearly depict certainfeatures. Such labeling and drawing practices do not necessarilyimplicate an underlying substantive purpose. As stated above, thespecification is intended to be taken as a whole and interpreted inaccordance with the principles of the invention as taught herein andunderstood to one of ordinary skill in the art.

As noted above, embodiments of the present disclosure relate to wirelessaudio system and device communication using OFDMA, and methods andsystems for baseband synchronization between devices of the wirelessaudio system. In order for a subscriber device to properly communicatewith an access point, the subscriber device must be able to compensatefor any frequency offset relative to the access point, so that frame andsymbol timing is maintained. In some examples, this may be done byfrequency and phase locking between the access point and subscriberbaseband clocks. The baseband clocks may be used by the access point andthe subscriber to transmit and receive data (i.e., audio data, controlsignals, pilot signals, etc.) via their respective antennas, sampleinput audio, and for various other purposes.

Frequency and phase locking between the baseband clocks of the wirelessaudio devices may be facilitated by having the access point and the oneor more subscribers generate all their respective baseband clocks from acommon reference oscillator. Each subscriber can tune its own referenceoscillator based on a phase offset measurement from any one of thebaseband clocks (even from other subscribers) in order to maintain afrequency and phase lock with the access point reference. This resultsin all of a given subscriber's baseband clocks being locked to therespective access point references, because all the baseband clocks aregenerated from the common reference.

In embodiments of the present disclosure, a subscriber may tune itsreference frequency (i.e., reference oscillator) based on sample clocktiming (i.e. phase) offset measurements between the subscriber and anaccess point, that are taken for each frame during communication betweenthe access point and the subscriber. Timing offsets between thetransmission sample clock and the reception sample clock in an OFDMAsystem result in a channel phase slope when plotted in the frequencydomain. This slope is proportional to the sample clock timing offset. Assuch, by determining the slope of the channel phase offset between atransmitted signal (from the access point) and a received signal (at thesubscriber), the delay, and thus a timing offset measurement, can bedetermined. This timing offset measurement can then be used to tune thesubscriber reference oscillator. The tuned reference oscillator can thenbe used to control the antenna data clock and the audio sample clock ofthe subscriber, thereby enabling the subscriber and access pointbaseband clocks to be in sync. And by making a determination of thetiming offset measurement for every frame, the reference oscillator ofthe subscriber audio device can be continuously tuned to maintain a“steady state” synchronization. Further, by controlling both the antennadata clock and the audio sample clock of the subscriber audio devicebased on the reference oscillator, both clocks will be in sync with eachother and with the corresponding clocks of the access point.

FIG. 1 illustrates an example simplified block diagram of a wirelessaudio communication system or environment 100 in which the methods andapparatuses of the present disclosure may be used. The wireless audiocommunication system may include an access point 110 and a plurality ofsubscriber devices 120A-N.

The access point 110 may be any suitable computing device, and mayinclude a processor, memory, antenna, and/or one or more other signalprocessing or computing components. In some examples, access point 110may be an auto mixer, laptop or desktop computer, or any other devicethat is configured to communicate with various other devices (e.g., aplurality of wireless audio devices), including subscriber devices120A-N.

The access point 110 may be configured for performing a variety offunctions or acts, such as those described in this disclosure (andaccompanying drawings). The access point 110 may include variouscomponents, including for example, a processor and memory. The accesspoint 110 may also include a display, user interface, and/or one or moreother electronic components. The processor may include a general purposeprocessor (e.g., a microprocessor) and/or a special purpose processor(e.g., a digital signal processor (DSP)). The processor may be anysuitable processing device or set of processing devices such as, but notlimited to, a microprocessor, a microcontroller-based platform, anintegrated circuit, one or more field programmable gate arrays (FPGAs),and/or one or more application-specific integrated circuits (ASICs). Thememory may be volatile memory (e.g., RAM including non-volatile RAM,magnetic RAM, ferroelectric RAM, etc.), non-volatile memory (e.g., diskmemory, FLASH memory, EPROMs, EEPROMs, memristor-based non-volatilesolid-state memory, etc.), unalterable memory (e.g., EPROMs), read-onlymemory, and/or high-capacity storage devices (e.g., hard drives, solidstate drives, etc.). In some examples, the memory includes multiplekinds of memory, particularly volatile memory and non-volatile memory.The memory may be computer readable media on which one or more sets ofinstructions, such as the software for operating the methods of thepresent disclosure, can be embedded. The instructions may embody one ormore of the methods or logic as described herein. For example, theinstructions reside completely, or at least partially, within any one ormore of the memory, the computer readable medium, and/or within theprocessor during execution of the instructions.

The terms “non-transitory computer-readable medium” and“computer-readable medium” include a single medium or multiple media,such as a centralized or distributed database, and/or associated cachesand servers that store one or more sets of instructions. Further, theterms “non-transitory computer-readable medium” and “computer-readablemedium” include any tangible medium that is capable of storing, encodingor carrying a set of instructions for execution by a processor or thatcause a system to perform any one or more of the methods or operationsdisclosed herein. As used herein, the term “computer readable medium” isexpressly defined to include any type of computer readable storagedevice and/or storage disk and to exclude propagating signals.

In some examples, access point 110 may be a base station, centralizedcontroller, or other computing device configured to communicate withmultiple wireless audio subscriber devices at the same time. Forexample, the access point may operate in a conference room, and thesubscriber devices maybe a plurality of microphones that communicatewith the access point, to provide a conferencing environment. Otherexamples are possible as well.

The access point 110 may include one or more antennas that enablewireless audio communication with the one or more subscriber devices120A-N, as well as a reference oscillator. The reference oscillator maybe used to control one or more baseband clocks of the access point. Forexample, an antenna sample clock may be controlled based on thereference oscillator, so as to control the timing of transmission andreception of various data.

The access point 110 may be configured to transmit data to one or morewireless audio subscriber devices 120A-N in a variety of formats andusing a variety of communication protocols. For instance, the accesspoint 110 may transmit data using an OFDMA scheme, wherein data istransmitted in frames. Each frame may include a plurality ofsubcarriers, some of which are used to transmit data, some of which arepilot subcarriers used to synchronize the access point with thesubscriber, and some of which are “guard” subcarriers to protect againstinterferences from adjacent channels or sub-channels in the frequencyspectrum. In one example, a given channel may be broken into 64different subcarriers. The channel may include 52 data subcarriers, 4pilot subcarriers, and 8 guard subcarriers. These numbers are used forexample only, and it should be appreciated that other numbers may beused as well.

Each pilot subcarrier may be transmitted at a specific and knownfrequency, and may be configured to not carry any audio or controlinformation. The known frequency location of each pilot subcarrier mayenable the subscriber that receives the frame to determine a phaseshift, and thus a timing offset between the access point 110 and thesubscriber. This is discussed in further detail below.

The wireless audio subscriber devices 120A-N may be portable wirelessaudio receivers, microphones, conference systems, speakers, and/or anyother devices that can be communicatively coupled to the access point110. The embodiments disclosed herein are described with reference tothe subscriber devices each being a microphone, however, it should beappreciated that the concepts and features disclosed herein may beapplied to other types of subscriber devices as well.

Each wireless audio subscriber device 120A-N may include one or moreantennas, a reference oscillator, a baseband clock for antenna/symboltransmission and reception via the antenna, a baseband clock for audiosampling, and appropriate processing and memory components to carry outthe functions described herein, particularly the signal processingfunctions described with respect to FIGS. 2 and 3. Specificallyregarding the processor and/or memory of the subscriber devices 120A-N,the disclosure herein with respect to the processor and/or memory of theaccess point 110 also applies to each subscriber device 120A-N.

The antenna(s) of each wireless audio subscriber device may operatebased on an antenna data clock, which determines the rate at which datais sampled from the antenna. The reference oscillator is used by thesubscriber device to control the various baseband clocks, which caninclude the antenna data clock. The wireless audio subscriber devicealso includes a baseband clock for audio sampling, which determines therate at which an incoming audio signal into the microphone is sampled.

FIG. 2 is a simplified signal process flow diagram 200 of an examplewireless audio subscriber device of the system of FIG. 1, in accordancewith some embodiments of the present disclosure. Any one or more of thesubscriber devices 120A-N may include the components and functionalitydescribed with respect to FIG. 2.

Diagram 200 illustrates an antenna 202, a radio frequency (RF) receiver204, an analog to digital converter (ADC) 206, a Fourier transform block(FFT block) 208, a sample timing offset estimator 210, aproportional-integral controller 212, a reference oscillator 218, firstand second phase locked loops 220 and 222, and an audio sample clock224.

The antenna 202 may be a single antenna, or may include a plurality ofantennas. The plurality of antennas may be arranged in an array. The RFreceiver 204 may be configured to detect OFDMA signals.

The ADC 206 may be configured to receive signals from the RF receiver.The ADC 206 may also be called a sampler, because the ADC is configuredto sample the input signal at a particular rate. The sampling rate isdetermined based on the antenna data clock, which is determined based onthe reference oscillator 218 as described below. The FFT block 208 isconfigured to convert the sampled input signal from the ADC 206 into thefrequency domain.

Where a wireless audio subscriber includes two or more antennas, eachantenna may have a corresponding RF receiver (204), ADC (206), and FFTblock (208). The output of the FFT block for each antenna may feed intothe sample timing offset estimator 210.

The sample timing offset estimator 210 is configured to receive theoutput of the FFT block 208 (i.e., a frame) and determine a time offsetestimate between the access point 110 and the subscriber. This isdescribed in more detail with respect to FIG. 3.

FIG. 3 illustrates a simplified process flow diagram 300 of the sampletiming offset estimator 210. The sample timing offset estimator 210takes in the frame 302 as input from the FFT block 208. It should beunderstood that the sample timing offset estimator 210 is configured toreceive as input multiple frames corresponding to multiple antennas, andprocess each frame as disclosed herein. The frame 302 includes aplurality of resource blocks 304A-N, distributed across the bandwidth ofthe frame 302. Each resource block has two pilot subcarriers (e.g.,pilot subcarriers 310 a and 310 b), each having an expected frequency.The sample timing offset estimator 210 first determines a channelestimate for both pilot subcarriers in the resource block, and thencomputes the channel phase slope by multiplying the channel estimate atsubcarrier k+D 310B by the complex conjugate of the channel estimate atsubcarrier k 310A. The sample timing offset estimator 210 repeats thiscomputation over all resource blocks 304A-N (and their correspondingpairs of pilot subcarriers) and accumulates the resulting channel phaseslope values, in order to filter out noise and the impact offrequency-selective fading.

The sample timing offset estimator 210 is also configured to estimatethe sample timing offset over a plurality of antennas, where thesubscriber includes two or more antennas. The same process of computingchannel estimates for the pilot subcarriers and multiplying the channelestimates within each resource block is repeated using the FFT outputfor the additional antenna(s), and the resulting channel phase slopevalues are accumulated with the sum from the first antenna.

The slope of the channel phase (in the frequency spectrum) isproportional to the timing offset between the access point and thesubscriber. By scaling the accumulated channel phase slope, the timingoffset in samples can be estimated for the frame 302. The timing offsetestimate is then output by the sample timing offset estimator 210.

The PI controller 212 receives the timing offset estimate. The PIcontroller includes a weighted integration of previously determinedtiming offset estimates 214, and a weighted current timing offsetestimate 216. The PI controller can adjust the weights based on atrade-off between fast initial convergence on an offset between theaccess point 110 and the subscriber, and a smooth steady stateoperation. Heavily weighting the most recent timing offset providesfaster convergence, but makes the system susceptible to transient noiseand disruptions. Using a lesser weight for the most recent timing offsetprovides slower convergence, but a more smooth change during steadystate operation of the PI controller, thus making the system lesssusceptible to rapid changes and noise. The PI controller 212 outputs atuning value to be used by the reference oscillator 218.

The reference oscillator 218 takes in the tuning signal output by the PIcontroller. The tuning signal is used to modify the referenceoscillator, so as to reduce the timing offset between the access point110 and the subscriber.

The output of the reference oscillator 218 is passed to two phase lockedloops (PLLs) 220 and 222. The first PLL 220 generates a baseband clockfrequency signal for the audio sample clock 224. The second PLL 222generates a baseband clock frequency signal for the ADC 206, so as tocontrol the sampling of the antenna.

The audio sample clock 224 gathers audio data where the subscriberdevice includes a microphone. Using the same reference oscillator toprovide baseband clock signals for the audio sample clock and thetransmission/reception of data via the ADC 206 enables the system toreduce issues of latency and eliminate the need for dropping or addingsamples, as well as providing other operational benefits.

FIG. 4 illustrates a flowchart of an example method 400 according toembodiments of the present disclosure. Method 400 may enable a wirelessaudio subscriber device to adjust its reference oscillator, so as tosynchronize its baseband clocks with an access point. The flowchart ofFIG. 4 is representative of machine readable instructions that arestored in memory and may include one or more programs which, whenexecuted by a processor may cause one or more systems or devices tocarry out one or more functions described herein. While the exampleprogram is described with reference to the flowchart illustrated in FIG.4, many other methods for carrying out the functions described hereinmay alternatively be used. For example, the order of execution of theblocks may be rearranged or performed in series or parallel with eachother, blocks may be changed, eliminated, and/or combined to performmethod 400. Further, because method 400 is disclosed in connection withthe components of FIGS. 1-3, some functions of those components will notbe described in detail below.

Method 400 starts at block 402. At block 404, method 400 includesreceiving a frame from an access point. As noted above with respect toFIG. 3, the frame may include a plurality of subcarriers, such as datasubcarriers, pilot subcarriers, and more.

At block 406, method 400 may include determining channel estimates forthe pilot subcarriers of the frame. The channel estimates are then usedat block 408 to determine the channel phase slope for the pair of pilotsubcarriers in each resource block. At block 410, method 400 includessumming the phase shifts for all pairs of pilot subcarriers, and acrossall the antennas used by the subscriber.

At block 412, method 400 may include determining a timing offsetestimate based on the sum of the channel phase slope values for eachresource block and each antenna. This timing offset estimate is measuredin samples.

At block 414, method 400 may include determining a tuning value for thesubscriber reference oscillator based on the timing offset estimate.This can include passing the timing offset estimate through aproportional-integral controller, which adds weights to the currenttiming offset, and an integration of past timing offsets. This enablesthe subscriber to reach a fast convergence in some scenarios, whileenabling smooth steady-state operation as well.

At block 416, method 400 may include modifying the reference oscillatorbased on the tuning value. The reference oscillator may provide areference frequency that is used to control one or more baseband clocksof the subscriber.

At block 418, method 400 may include controlling an antenna data clockand an audio sample clock based on the reference oscillator, which wasmodified by the tuning value. Controlling both the audio sample clockand the antenna data clock based on the same reference oscillatorfrequency enables the subscriber to reduce latency and avoid issued fromhaving to insert or drop audio samples. Method 400 may then revert backto block 404, and receive a next frame from the access point. The methodmay be repeated to form a steady-state feedback loop, ensuring that theaudio sample clock and antenna data clock remain in sync with thebaseband clocks of the access point. Method 400 may then end at block420.

Any process descriptions or blocks in figures should be understood asrepresenting modules, segments, or portions of code which include one ormore executable instructions for implementing specific logical functionsor steps in the process, and alternate implementations are includedwithin the scope of the embodiments of the invention in which functionsmay be executed out of order from that shown or discussed, includingsubstantially concurrently or in reverse order, depending on thefunctionality involved, as would be understood by those having ordinaryskill in the art.

This disclosure is intended to explain how to fashion and use variousembodiments in accordance with the technology rather than to limit thetrue, intended, and fair scope and spirit thereof. The foregoingdescription is not intended to be exhaustive or to be limited to theprecise forms disclosed. Modifications or variations are possible inlight of the above teachings. The embodiment(s) were chosen anddescribed to provide the best illustration of the principle of thedescribed technology and its practical application, and to enable one ofordinary skill in the art to utilize the technology in variousembodiments and with various modifications as are suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the embodiments as determined by the appendedclaims, as may be amended during the pendency of this application forpatent, and all equivalents thereof, when interpreted in accordance withthe breadth to which they are fairly, legally and equitably entitled.

1. A method for baseband clock synchronization in a wireless microphonesystem comprising: receiving a plurality of pilot subcarriers from anaudio transmitter; determining a timing offset estimate based on thepilot subcarriers, wherein determining the timing offset estimatecomprises: determining a channel phase slope based on the plurality ofpilot subcarriers; and determining the timing offset estimate based onthe channel phase slope; determining a tuning value by passing thetiming offset estimate through a proportional-integral controller;determining a modified reference signal by modifying a referenceoscillator based on the tuning value; and controlling (i) an audiosample clock and (ii) an antenna data clock based on the modifiedreference signal.
 2. The method of claim 1, further comprising:receiving a plurality of frames, each frame including the plurality ofpilot subcarriers.
 3. The method of claim 2, further comprisingdetermining the tuning value for each frame.
 4. The method of claim 3,further comprising; determining the modified reference signal for eachframe; and updating the audio sample clock and the antenna data clockeach frame.
 5. (canceled)
 6. The method of claim 1, wherein determiningthe channel phase slope comprises: determining a channel estimate foreach of the plurality of pilot subcarriers; determining one or morechannel phase slope values corresponding to one or more pairs of theplurality of pilot subcarriers; and summing the one or more channelphase slope values.
 7. The method of claim 1, wherein determining thetuning value comprises applying weighted coefficients to (i) the timingoffset estimate and (ii) an integration of previously determined timingoffset estimates.
 8. A wireless audio microphone system comprising: anaudio transmitter configured to: transmit a plurality of pilotsubcarriers; and one or more audio receivers, each configured to:receive the plurality of pilot subcarriers; determine a timing offsetestimate based on the plurality of pilot subcarriers; determine a tuningvalue by passing the timing offset estimate through aproportional-integral controller, wherein determining the tuning valuecomprises applying weighted coefficients to (i) the timing offsetestimate and (ii) an integration of previously determined timing offsetestimates; determine a modified reference signal by modifying areference oscillator based on the tuning value; and control (i) an audiosample clock and (ii) an antenna data clock based on the modifiedreference signal.
 9. The wireless audio system of claim 8, wherein theaudio transmitter is further configured to transmit a plurality offrames, each frame including the a respective plurality of pilotsubscribers, and wherein the one or more audio receivers are eachfurther configured to receive the plurality of frames.
 10. The wirelessaudio system of claim 9, wherein the one or more audio receivers areeach further configured to determine the tuning value for each frame.11. The wireless audio system of claim 10, wherein the one or more audioreceivers are each further configured to; determine the modifiedreference signal for each frame; and update the audio sample clock andthe antenna data clock each frame.
 12. The wireless audio system ofclaim 8, wherein the one or more audio receivers are each furtherconfigured to determine the timing offset estimate by: determining achannel phase slope based on the plurality of pilot subcarriers; anddetermining the timing offset estimate based on the channel phase slope.13. The wireless audio system of claim 12, wherein the one or more audioreceivers are each further configured to determine the channel phaseslope by: determining a channel estimate for each of the plurality ofpilot subcarriers; determining one or more channel phase slope valuescorresponding to one or more pairs of the plurality of pilotsubcarriers; and summing the one or more channel phase slope values. 14.(canceled)
 15. An audio receiver of a wireless audio system, the audioreceiver comprising: an antenna configured to receive a plurality ofpilot sub carriers from an audio transmitter; and circuitry configuredto: determine a timing offset estimate based on the plurality of pilotsubcarriers; determine a tuning value based on the timing offsetestimate by applying weighted coefficients to (i) the timing offsetestimate and (ii) an integration of previously determined timing offsetestimates; determine a modified reference signal based on the tuningvalue; and control (i) an audio sample clock and (ii) an antenna dataclock based on the modified reference signal.
 16. The audio receiver ofclaim 15, wherein the antenna is further configured to receive aplurality of frames, each frame including the plurality of pilotsubcarriers, and wherein the circuitry is further configured todetermine the tuning value for each frame.
 17. The audio receiver ofclaim 16, wherein the circuitry is further configured to: determine themodified reference signal for each frame; and update the audio sampleclock and the antenna data clock each frame.
 18. The audio receiver ofclaim 15, wherein the circuitry is further configured to determine thetiming offset estimate by: determining a channel phase slope based onthe plurality of pilot subcarriers; and determining the timing offsetestimate based on the channel phase slope.
 19. The audio receiver ofclaim 18, wherein the circuity is further configured to determine thechannel phase slope by: determining a channel estimate for each of theplurality of pilot subcarriers; determining one or more channel phaseslope values corresponding to one or more pairs of the plurality ofpilot subcarriers; and summing the one or more channel phase slopevalues.
 20. (canceled)